1. Field of the Invention
The present invention relates to a medical image processing apparatus and a computer-readable storage medium.
2. Description of Related Art
In diagnosis of a thoracic portion, a thoracic portion X-ray image captured in the past is compared with the present thoracic portion X-ray image, and this is called comparative interpretation.
In order to assist comparative interpretation, there is a well-known technique where a computer obtains a difference between the image captured in the past and the present image to reduce a normal structure where there is no temporal change (anatomical structure of the human body), and the computer generates a difference image emphasizing portions where there is temporal change such as an abnormal shadow.
However, the thoracic portion X-ray image projects a thoracic portion of the human body with three dimensions in two dimensions. Therefore, compared to FIG. 17A with correct posture, when the subject tilts forward or backward, or rotates around the body axis as shown in FIG. 17B or there is a misalignment in the position of the X-ray source (oblique incidence) as shown in FIG. 18 when the subject is captured, the image is projected in a state where the positions of the bone, blood vessel and other structures in the lung field are misaligned from the state in the correct posture. Especially in small medical institutions such as general practioners, a fixed portion is not captured as in mass examination and the X-ray source is often moved. Moreover, since there is no specialized capturing operator, the positioning may be varied, and a misalignment of the positions of the above structures easily occur. When the positions of the above structures in the lung field are misaligned in the 2 images, the structure which is normal may also be drawn when simply the difference is obtained, and this becomes an artifact.
Conventionally, in order to correct the misalignment of the positions between 2 images which are compared, local matching processing, warping processing, etc. are performed to match the positions of the images and then the difference is obtained. However, in 2 images where the positions of the structure are misaligned, the positions do not match with any structure even if positioning is performed, the normal structure is drawn in the entire difference image, and the image becomes very difficult for the physician to interpret.
For example, Japanese Patent No. 3773111 describes the following technique. A point on the target corresponding to points on the 2 images is obtained, and the degree of forward tilt/backward tilt is estimated from the result. Then, the distance between specific portions in the projected image is measured and the degree around the vertical axis is estimated based on the above. One of the images is deformed based on the estimated degree and depth, and the difference image is generated.
According to the technique described above, the degree of forward tilt/backward tilt and the degree around the body axis is estimated and the processing is performed. Therefore, the misalignment of the position due to the forward tilt, the backward tilt and the rotation around the body axis can be corrected. However, the misalignment of the position due to oblique incidence cannot be corrected. Therefore, when the oblique incidence occurs, the artifact in the region drawing the attention of the physician cannot be removed. Moreover, according to the above technique, the tilt of the body is estimated based on information of one portion of the image. Therefore, there tends to be a mistake and it is not possible to stably provide the difference image where the artifact is reduced in the region drawing the attention of the physician.